The present invention relates to a read-only memory and a method of manufacturing the same and, more particularly to a cell structure of a high-density read-only memory using a vertical MOSFET and a method of manufacturing the same.
In a conventional mask ROM using a MOSFET, a current ON/OFF state and the presence/absence of a MOSFET are used to store data of logic "1" or "0". For example, contact hole and threshold voltage write type ROMs with different margins of channel doping or different gate oxide film thicknesses are used in practice. On the other hand, in a conventional contact hole write type ROM, the data write process is performed at the end of the entire process in favor of short turn-around time of ROM development, thereby achieving low-cost mask ROMs. A conventional threshold voltage write type ROM requires 0.5 contact hole/bit, while a conventional contact write type ROM requires one contact hole/bit. The threshold voltage write type ROM requires a small number of contact holes as compared with that of the contact hole write type ROM. Therefore, the memory cell size of the threshold voltage write type ROM is smaller than that of the contact hole write type ROM. For example, the threshold voltage write type ROM is exemplified by a VMOSTr described in IEEE Vol. SC-11 No. 5, Oct. 1976, P. 614.
Although the above-mentioned two types of ROMs have respective advantages, an element isolation region must be formed between each adjacent memory cells so as to electrically isolate them from each other. In contact writing, for example, an area of a unit memory cell which includes an element isolation region is about 10 .mu.m.sup.2 when a pattern size is 1.0 .mu.m and an overlay accuracy is 0.2 .mu.m. When a pattern size is about 0.5 .mu.m and an alignment precision is about 0.1 .mu.m, the area of the unit memory cell is about 6 .mu.m.sup.2. Therefore, contact writing is not suitable for a large scale integration or a high packing density.